In view of the decreasing amount of oil reserves which constitute the raw material for the production of short-chain hydrocarbons and derivatives thereof, alternative processes for the production of such base chemicals are of a growing importance. In such alternative processes for the production of short-chain hydrocarbons and derivatives thereof, often highly specific catalysts are used therein for converting other raw materials and/or chemicals to hydrocarbons and their derivatives such as in particular short-chain olefins. A particular challenge involved in such processes not only relies in the optimal choice of reaction parameters but, more importantly, in the use of particular catalysts allowing for the highly efficient and selective conversion to a desired hydrocarbon or derivative thereof such as in particular olefinic fractions. In this respect, processes in which methanol is employed as the starting material, are of particular importance, wherein their catalytic conversion usually leads to a mixture of hydrocarbons and derivatives thereof, in particular olefins, paraffins, and aromatics.
Thus, the particular challenge in such catalytic conversions resides in the optimization and the fine tuning of the catalysts employed as well as the process architecture and parameters such that as high a selectivity towards as few products as possible may be achieved. For this reason, such processes are often named after the products for which a particularly high selectivity may be achieved in the process. Accordingly, processes which have been developed in the past decades towards the conversion of oxygenates to olefins and in particular of methanol to olefins which have gained increasing importance in view of dwindling oil reserves are accordingly designated as methanol-to-olefin-processes (MTO-processes for methanol to olefins).
Among the catalytic materials which have been found for use in such conversions, zeolitic materials have proven of high efficiency, wherein in particular zeolitic materials of the pentasil-type and more specifically those having an MFI-and MEL-type framework structures including such zeolites displaying an MFI-MEL-intergrowth type framework structure are employed. As regards the specific application of zeolitic materials and in particular zeolitic materials of the pentasil-type in catalysis and more particularly in processes for the conversion of oxygenates to olefins such as the MTO-processes discussed in the foregoing, EP 2 460 784 A1 relates to a method for producing propylene from an oxygen-containing compound using a catalyst which can keep its stable activity for a prolonged period of time in the manufacturing process. DD 238 733 A1 relates to a synthetic procedure for the preparation of selective olefin catalysts. McIntosh et al. in Applied Catalysis 1983, vol. 6, pp. 307-314 concerns the properties of magnesium and zinc oxide treated ZSM-5 catalysts for the conversion of methanol into olefin-rich products. Likewise, Ciambelli et al. in “Acid-Base Catalysis in the Conversion of Methanol to Olefins over Mg-Modified ZSM-5 Zeolite”, Successful Design of Catalysts, edited by T. Inui, Elsevier Science Publishers B.V., Amsterdam 1988 investigates the effect of magnesium in pure and bonded ZSM-5 catalysts, in particular on the acid-base properties and their effect on olefin selectivities in the MTO-process.
In the aim of further improving the properties of such catalysts, their further treatment with specific compounds has been investigated, wherein in particular the microporous system typical of these zeolitic materials may be loaded with different compounds. Thus, Okado et al. in Applied Catalysis 1988, vol. 41, pp. 121-135 concerns the deactivation resistance of ZSM-5-type zeolites containing alkaline earth metals and their use in the conversion of methanol. Similarly, Goryainova et al. in Petroleum Chemistry 2011, vol. 51, pp. 169-173 investigates on magnesium-containing zeolite catalysts for the synthesis of lower olefins from dimethyl ether. U.S. Pat. No. 4,049,573, on the other hand, concerns zeolite catalysts containing oxides of boron or magnesium.
On the other hand, as regards the synthesis of zeolitic materials in general, efforts have been invested into their optimization for economical and increasingly also for environmental reasons. In this respect, it has been found that crystallizing an aluminosilicate in the absence of an alkali source allows to omit the ion-exchange procedures normally required after crystallization to obtain the so called H-form thereof, wherein the alkali metals present in the resulting material as non-framework element are exchanged against protons. The ion exchanges necessitate additional steps in the manufacturing process considerably reducing the space-time-yield of the zeolite, generating high volumes of waste water, consuming energy and thus increasing overall production costs. Alkali-free synthetic methodologies are thus highly beneficial as it makes the synthesis process simpler with fewer steps, thus more economical and industrially viable. Such a manufacturing process also generates less waste during catalyst production.
Thus, Liu et al. in Chemistry Letters 2007, vol. 36, pp. 916 and 917, for example, concerns a synthetic procedure for the preparation of MWW-type metallosilicates under alkali-free conditions. The De Baerdemaeker et al. in Microporous and Mesoporous Materials 2011, vol. 143, pp. 477-481 concerns the synthesis of MTW-type zeolites which is performed in an alkali-free and fluoride-free synthetic procedure. In Takeguchi et al. in Journal of Catalysis 1998, vol. 175, pp. 1-6 the synthesis of alkali-free Ga-substituted MCM-41 catalysts is described. Ahedi et al. in Journal of Materials Chemistry 1998, vol. 8, pp. 1685-1686 concerns the synthesis of FER titanosilicates from a non-aqueous alkali-free seeded system. Dodwell et al. in Zeolites 1985, vol. 5, pp. 153-157 concerns the crystallization of EU-1 and EU-2 in alkali and alkali-free systems. Shibata et al. in Applied Catalysis A: General 1997, vol. 162, pp. 93-102, on the other hand, describes routes for the synthesis of alkali-free MFI borosilicates.
Furthermore it is now known that the formation, in particular the diameter, of the zeolite crystals obtained via alkali-free processes can be tuned by adjusting the temperature, stirring rate, concentration of the synthesis mixture and the duration of the crystallization. This may be of importance to adjust the diffusion properties of the zeolite for specific catalytic applications and to allow for optimal shaping and properties of the resulting shaped bodies. In particular, appropriate shaped bodies often need to be prepared prior to the introduction of the catalyst into a reactor to carry out the catalytic transformation.
In this respect, DE 103 56 184 A1 relates to a zeolitic material of the pentasil type having a molar ratio of Si to Al of from 250 to 1500, wherein furthermore at least 90% of the primary particles of the zeolitic material are spherical, wherein 95% by weight thereof have a diameter of less than or equal to 1 μm. Furthermore, said document discloses a specific treatment of ZSM-5 powder with demineralized water under autogeneous pressure, wherein it is taught that both the activity and the selectivity would be improved by the water treatment of the ZSM-5 powder under hydrothermal conditions when employed in a process for the preparation of tetraethylenediamine from piperazine and ethylenediamine. DE 41 31 448 A1 on the other hand concerns essentially alkali-free borous silicate crystals having a zeolite structure and a size from 2 to 150 μm.
Reding et al. in Microporous and Mesoporous Materials 2003, vol. 57, pp. 83-92 investigates on synthetic procedures for obtaining nano-crystalline zeolite ZSM-5. Likewise, Van Grieken in Microporous and Mesoporous Materials 2000, vol. 39, pp. 135-147 investigates the crystallization mechanism in the synthesis of nanocrystalline ZSM-5. Rivas-Cardona in Microporous and Mesoporous Materials 2012, vol. 155, pp. 56-64, on the other hand, investigates silicalite-1 precursor mixtures having varying degrees of dilution.
Despite the considerable efforts related by the prior art relative to the synthesis of novel zeolitic materials on the one hand by using new and improved synthetic procedures, and their various applications such as in particular in the field of catalysis on the other hand, there remains an ongoing need to provide new zeolitic materials displaying yet further improved properties in particular with respect to the large and constantly increasing number of applications in which they may be employed and in particular in the very important field of catalytic processes.